The present invention relates to programmable logic controllers and specifically to such devices applied to analog inputs under the control of a logic based program control sequence.
Programmable logic controllers (PLCs) are specialized miniature computers employed to observe electromechanical device activities, convert activities into data models, apply behavioral parameters from a logical perspective, and based upon variables considered, control external electromechanical devices such as solenoids, relays, indicating lamps, or the like. PLCs interface electromechanical devices with logic based program control sequences for monitoring, controlling, and enunciating activities specific to an application.
PLC devices are differentiated by the way inputs are sampled and processed. From an input perspective, PLC devices are applied to continuous (real-time) sampling for feedback governing processes or utilized to sample static (snapshot) conditions processed by logic based control sequences. Continuous sampling devices invoke real-time processes for generating controlled outputs in direct proportion to sampled inputs. Such an activity could be comprised of an anti-skid braking system, a servo motor operation, a Computer Numeric Controlled (CNC) machining process, or the like. Static sampling devices on the other hand, invokes a process cycle judging one state against another from a sequence of logic based control functions. As a result, outputs are generated and the process is incremented to a whole new cycle of unrelated control functions. In practice, continuous and static sampling activities from a PLC input perspective, employ separate but distinct process methodologies inclusive of independent hardware and software design approaches.
U.S. Pat. No. 4,831,320, filed May 16, 1989, by Koji Takata, and entitled DUTY FACTOR CONTROL METHOD, depicts a control method for electromechanical actuators of equally discrete intermediate positions. Classic of feedback governing activities, a pulse width control methodology is described to sample ratios between durations of mutually discrete electromechanical states for approximating intermediate conditions. By counting the number of opposing single-bit state changes produced by an electromechanical device and dividing the count by a period of time, an intermediate change in distance, flow, or the like, is estimated. Accordingly, sensor devices producing an infinite number of non-discrete intermediate positions such as rheostats, resistive thermal sensors, or the like, do not generate discrete opposing states that can be counted by the methodology. Besides real-time demands on hardware and software resources to support continual approximations, a sampling means of the methodology fails to facilitate non-discrete intermediate state changes produced by analog sensor devices.
A device such as the sequence type controller depicted in U.S. Pat. No. 4,224,530, filed Sep. 23, 1980, by Robert J. Simcoe, and entitled TIME TO GO AND DIAGNOSTIC DISPLAY FOR ELECTRONIC SEQUENCE TYPE APPLIANCE CONTROLLER, illustrates an electromechanical motorized timer replacement but more importantly, a sequential logic based program control device with static sampling of inputs is presented. Inputs from manually actuated switches and time to go states corresponding to the electronic equivalent of a motorized timer advances a control sequence to a whole new (conditional) process step or incremental (process) state. The incremental process of the control sequence is restricted to mutually discrete input signals representing one of two opposing states.
Simcoe teaches a method and system for an electronic sequence type controller for appliances or the like that provide numerical indications while executing program cycles which a user has selected. As presented, numerical representations produced by a sequential counter indicates a particular control cycle or cycle duration. When input conditions of a particular control cycle are satisfied, the control process is incremented to the next cycle while providing a visual means to follow the process. While a visual display advances or stops at a particular value, the electrical state of all switches and time to go timer conditions responsible for the current control cycle count assume validity. Although a diagnostic feature of the display is presented, actual input state conditions are not directly indicated numerically or by any other display means. Furthermore, input conditions are limited by two discrete opposing states for advancing the incremental process illustrated. Intermediate conditions of an input representing values other than two opposing states are entirely disassociated from a display means, processing means, or diagnostic means presented.
A software development tool used to identify and describe specific I/O points in a PLC system is described in U.S. Pat. No. 5,613,115, filed Dec. 9, 1991, Nicholas T. Gihl and John R. Skach, and entitled METHOD FOR USING PLC PROGRAMMING INFORMATION TO GENERATE SECONDARY FUNCTIONS SUCH AS DIAGNOSTIC AND OPERATOR INTERFACE. A user-developed control program with various diagnostic, status, alarm, and user control functions is generated from descriptive comments to operate PLC devices. The Prior Art depiction illustrates the use of electromechanical devices such as push buttons, limit switches, relays, or the like, to satisfy input requirements of a logic based program control sequence. The methodology fails to identify or provide a process means or a control means to recognize, specify, or enable electromechanical devices possessing an infinite number of non-discrete intermediate (analog) state conditions such as resistive, voltaic, or current operated devices deriving more than two discrete opposing logic states. Clearly, analog inputs were never intended to be used with the Prior Art methodology depicted because the process means fails to characterize intermediate non-discrete states as a variable of logic based program control sequences.
Because PLC devices are valued for their ability to control electromechanical devices based upon user-defined input scenarios, extreme importance is focused upon legitimate input strategies. Fail-safe designs are adopted that recognize the potential for input sensor or sensor wiring failure. General practice assumes that an input circuit will break electrical contact leaving a predictable (fail-safe) logic state based upon the placement of current sourcing or current sinking resistors at the PLC input terminals. Fail-safe strategies for two-state sensor circuits neglect to isolate circuit failure from sensor activity. When the electrical contacts of a sensor device become damaged due to longevity, corrosion, abuse, neglect or the like, electrical resistance continues to increase over time. Likewise, damaged contact cycles may vary slightly in resistance from one actuation to another. Theoretically, a number of these cycles could go undetected resulting in process errors of a logic based program control sequence.
Since binary logic principles are by definition based on two discrete opposing states, signal levels outside the domain of (active) ones and (de-active) zeros must be converted to signals PLC devices recognize. Converters and I/O modules interface analog signals with logic based control sequences. Converters detect specific attributes from analog sources to generate logic states for PLC input requisites. Limited by hardware constraints targeted to specific tasks, converters are ridged pre-configured devices. Employing data bus protocols controlled by logic based program control sequences, highly adaptive I/O modules convert analog signals into digital (magnitude) values. Logic based program control sequences cannot utilize these raw data values without extensive algorithms to quantify, qualify, and compare against. For this reason, I/O modules consume significantly more hardware and software resources to manage. Defining magnitude values in terms pertaining to human perception is inherently more difficult to accomplish because numerical values represent abstract concepts.
Frequently, it is necessary to gather data such as pressure, temperature, fluid levels, or the like without the need to observe every increment of an infinite number of non-discrete intermediate values produced by voltage, current, or resistive sources. Often, it is sufficient to determine values and ranges such as high, full, medium, low, empty, or similar increments to enlist the benefits of logic based program control sequences. It would be more efficient, from a software and hardware resource perspective, to enlist a methodology that avoided timers and counters to determine ratios, values and ranges. Furthermore, a diagnostic means would be better served from a display means if both sensor and sensor wiring status could be presented. Additionally, it would be more useful to define input scenarios in descriptive terms pertaining to human perception for xe2x80x9creal-worldxe2x80x9d values employing relevant data such high, full, medium, low, empty, or similar increments to enlist non-discrete intermediate values for logic based program control sequences. Consequently, these obstacles among others are easily defeated by preferred embodiments of the present invention.
Accordingly, it is an object of the present invention to provide a sampling means to convert non-discrete intermediate (analog) values of voltage, current, or resistance into user-defined discrete incremental states to satisfy operational requirements of a logic based program control sequence.
Another object of the present invention is to provide a diagnostic feature of a display means to present sensor circuit status represented by user-defined incremental states to characterize sensor devices or circuit wiring.
Still another object of the present invention is to provide a process means to define resistive, voltaic, or current sources as incremental states to satisfy input requirements of a logic based control program sequence.
Yet another object of the present invention is to provide a diagnostic feature employing incremental states to identify electrical opens and shorts within sensor circuits for isolating circuit failure.
Another object of the present invention is to provide a user-defined filter means to select significant incremental state transitions for stabilizing sensor status.
Still another object of the present invention is to provide a process means to select specific incremental states for characterizing voltage, current, or resistance values.